#3010 Amazing Australian "Boulder" opal forms 1st from iron oxide (rust) rich mud and sand which solidifies and hardens over time. The brown and red colors  are the "ironstone" that develops. As it dries it shrinks and forms cracks and voids into which opal later flows into and fills. The best opal has the fiery iridescent rainbow of color found in precious opal. Boulder opal is very stable but is softer than quartz gems. It is still suitable for any form of jewelry including rings and bracelets if it is properly cared for.